Novelists are commonly suspected of basing their fictional characters on real people. Sometimes the singularity of a character is such that even the uninformed reader will suspect that there must be have been a real prototype. (Muriel Spark, we sense before we are told, must have got Miss Jean Brodie from life.) Yet only a certain kind of novel includes "real people" who, by some kind of celebrity, are already known to us - whose voices we have heard.

Andrew O'Hagan's Personality features real people in two ways. First, his story of singing star and anorexic Maria Tambini is based on the life of Lena Zavaroni, who became famous when she won the TV talent show Opportunity Knocks at the age of 13 and was for several years hugely popular on stage and television. A prefatory Note to the Reader concedes, elliptically, that the novel "bears a relation to the lives of several dead performers", but does not name the original of its protagonist. An acknowledgments section at the close lists O'Hagan's main sources, which include the local newspaper for the Scottish island of Bute (where Zavaroni was born and brought up), but mentions nothing to do with Zavaroni herself. While every review of O'Hagan's novel mentioned Tambini's real-life counterpart, not one of the many extracts from reviews that festoon the paperback edition actually names her.

So you are to know without ever being told. Other contemporary novelists (Joyce Carol Oates making a fictional protagonist out of Marilyn Monroe in Blonde, Don DeLillo taking us into the mind of Lee Harvey Oswald in Libra, for instance) have felt no such reticence. In the case of Personality, it seems to leave the novelist maximum room for invention while inviting readers to believe in the character's destiny.

It is of a piece with O'Hagan's calculated refusal to know quite what is going on in his protagonist's head. We know about her, but we do not know her. And fiction must veer from biography; facts are changed. Maria's father is an unnamed American sailor, once stationed in Scotland, who had a brief affair with her mother, Rosa. In reality, Lena Zavaroni was brought up by both her parents and was close enough to her father for the two of them to perform together in later years.

A second type of "real person" who appears in the novel is the supporting celebrity. "Showbusiness personalities enter the narrative under their own names," as the Note to the Reader puts it. Maria meets Bernie Winters, Eric and Ernie, Lucille Ball, even Ronald Reagan. Some of these "personalities" (a word that acquires a sardonic tone in the course of the novel) speak. There is Les Dawson in Maria's television dressing room, with a stream of bad puns and old-trouper gags to which many a reader will be able to put his voice.

Maria duly reacts as if she is "lost in mirth". "Personality" (the title of one of Zavaroni's hits) is indefatigably acted out. Here (as later, when Dean Martin or Johnny Carson speak to her) the point is that the celebrity speaks just as we - and Maria - would expect.

There are odd precedents in the history of the novel for the appearance of "showbusiness" characters. The actor David Garrick regularly appeared in 18th-century novels. Tobias Smollett gave speaking parts in his fiction to favoured actors and writers. The effect was to include novel readers within a circle of the cultured and knowing. Television has given some of O'Hagan's characters an aura of familiarity that makes their appearance in Personality disconcerting. The most prominent of them is Hughie Green, who has two chapters of dramatic monologue that artfully recall the gruesome phrase-making of his TV appearances. "Showbusiness. Before you call it an illness you want to think what it does for people... Showbusiness is glory in the afternoon and sunshine after dark." He is "always speaking in headlines" says Maria, half in admiration.

He keeps returning in the novel, eloquently platitudinous and self-serving, with a distinctive syntax of sincerity: "Believe me... Don't get me wrong... There. I've said it now." In this novel's neatest feat of characterisation, its version of Hughie Green is made grotesquely believable by virtue of his own performance of "personality".

· John Mullan is senior lecturer in English at University College London